Abstract
The emergence of resistance to imipenem by Pseudomonas aeruginosa was investigated with four pairs of isolates. Each pair represented pretherapy (susceptible) and posttherapy (resistant) specimens. In all cases, the imipenem-resistant isolates did not demonstrate changed susceptibilities to other beta-lactams. Agarose gel electrophoresis revealed no change in plasmid profiles between any pair of isolates. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the Sarkosyl-insoluble membrane protein revealed the loss of an outer membrane protein of apparent molecular mass 48 to 49 kilodaltons in posttherapy strains when grown with imipenem selection (5 micrograms/ml). There was no significant difference in the binding of [14C]imipenem to the penicillin-binding proteins of the pre- and posttherapy strains. Trichloroacetic acid precipitation of membranes isolated after growth in the presence of [14C]imipenem revealed that significantly less drug was bound to Sarkosyl-soluble membrane protein in three of the four posttherapy strains than the membrane proteins of the respective pretherapy strains. beta-Lactamase activity against imipenem at 100 or 3 microM was not detected in any isolate either with or without induction. These data suggest that resistance to imipenem is associated with the loss of a 48- to 49-kilodalton outer membrane protein accompanied by, in three of four cases, decreased penetration of the antibiotic across the outer membrane.